The number of moles in $4\times {10}^{31}$  atoms of iron are

$4\times {10}^{31}$  iron atoms $\to$  ? moles of iron

So, $\frac{1\text{mol}}{6.023\times {10}^{23}\text{atoms}}\times 4\times {10}^{31}\text{atoms}=6.64\times {10}^7\text{mol}$

Therefore, $4\times {10}^{31}$  atoms of iron has $6.64\times {10}^7\text{mol}$ of iron,(Fe).